Abstract

The decay rate of K L → π 0 γγ has been measured with the NA48 detector at the CERN SPS. A total of 2558 K L → π 0 γγ candidates have been observed with a residual background of 3.2%. The branching ratio is determined to be (1.36±0.03 (stat)±0.03 (syst)±0.03 (norm))×10 −6 and the vector coupling constant a v =−0.46±0.03 (stat)±0.04 (syst). This result suggests that the CP-violation effects are dominating in the K L → π 0 e + e − decay. An upper limit for the K L → π 0 γγ decay rate in the two photon mass region m γγ < m π 0 is also given.

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